Code Switching in French Diplomatic Correspondence during the Peace Negotiation at Rijswijk (1696-1697)

I reported in 2015 a practice of using two or three different codes in a single despatch in French diplomatic correspondence during the American Revolutionary War (see another article). As early as a century earlier, during the reign of Louis XIV, such code switching had been prescribed for French plenipotentiaries for the peace negotiation at Rijswijk, as noted in Jörg Ulbert, "Zur Verschlüsselung französischer Ministerialkorrespondenzen" in Geheime Post (2015) (n.21).

The Treaty of Rijswijk (Ryswick) signed in 1697 ended the War of the Grand Alliance (a.k.a. the Nine Years War) in which the aggression of Louis XIV was held back by the allied forces of England, the Emperor, the Dutch, Spain, etc. The peace, however, left unsolved the issue of the Spanish crown after the expected death of Charles II of Spain and, a few years later, France would fight against another Grand Alliance in the War of Spanish Succession.

As with other French codes at the time (see another article), the codes employed figures to represent single letters, syllables, and words. One had entries 1-500 and the other had entries 1-378. These two codes were intended to be used alternatively in the same message. They are given identifying numbers "379" and "720", which can be used to indicate to the recipient which code is used first.

The Memorandum for the French Plenipotentiaries at the Peace Congress of Rijswijk (1696) is worth quoting in English translation.

In order to prevent accidents which might occur in the event the letters in cipher are intercepted, it has been thought advisable to make two keys, which one who enciphers will take the trouble to use alternately in the same despatch, that is to say, he will always encipher five lines with one of the said keys, the sixth line will be of the other cipher, and so on alternately. And so that the one who has to decipher knows with which key the despatches he has to decipher are enciphered. The said keys have been numbered, one (379) and the other (720), so that when one begins to encipher a despatch, if it is with the key numbered (379), it will be necessary to put this character (379) for the first character of the despatch, which will make known that the first five lines will be ciphered with the key numbered (379); at the sixth line, it will be useless to put the character (720) to indicate the change of the cipher, because it is supposed as an inviolable thing that when one has enciphered five with one key, the other key will be used to the following five lines, and so on alternatively until the end of the despatch.

The prescribed switching is different from the indicator system at the time of the American Revolutionary War. Still, the evidence for the two periods suggests that code switching may have been a practice during the last century of the ancien regime.

©2018 S.Tomokiyo
First posted on 8 August 2018. Last modified on 8 August 2018.
Cryptiana: Articles on Historical Cryptography
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